The following is a list of suitable leafy greens you can purchase from your local green-grocers of try your hand at growing yourself. Leafy greens should make up at least 50% of the fresh food you provide your rabbit.
Greens with High Oxalic Acid Content
The following veggies have a really high oxalic acid content. They should be fed carefully in moderation, only ONE item from this category should be fed a day.
A Note on Cabbages
Cabbages are naturally very gassy foods, they should not be fed exclusively and should not make up more than half of the leafy green part of the diet. Introduce them very slowly and one at a time, most rabbits fed a natural high green diet can tollerate cabbages fine, HOWEVER some rabbits have a very low tollance for cabbages and they can cause bloat. Personally I avoid feeding cabbages to babies under 16 weeks old, but ALL my adult rabbits have no problems with them being included in their diet.